1. Technical Field
The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly, to a nitride based semiconductor device reducing leakage current and a method for manufacturing the same.
2. Description of the Related Art
In general, III-nitride based semiconductors including III-group elements such as gallium (Ga), aluminum (Al), and indium (In) and nitrogen (N) have characteristics such as a wide energy band gap, high electron mobility and saturated electron speed, and high thermal chemical stability. A nitride-based field effect transistor (N-FET) based on the III-nitride based semiconductors is manufactured based on semiconductor materials having wide energy band gap, for example, materials such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), aluminum indium gallium nitride (AlInGaN), etc.
The general nitride-based field effect transistor has a so-called high electron mobility transistor (hereinafter, referred to as ‘HEMT’) structure. For example, a semiconductor device having the HMET structure includes a base substrate, an epitaxial growth layer formed on the base substrate, and a Schottky electrode and an ohmic electrode disposed on the semiconductor layer. The nitride based semiconductor device generates 2-dimensional electron gas (2-DEG) used as a movement path of current in the semiconductor layer and may perform forward and reverse operations by using the 2-DEG as a carrier movement path.
Meanwhile, the epitaxial growth layer is formed by performing an epitaxial growth process by using the base substrate as a seed layer. For example, by using a sapphire substrate as the base substrate and performing the epitaxial growth process with respect to the sapphire substrate, a gallium nitride layer and an aluminum gallium nitride layer may be formed on the sapphire substrate. As a result, the base substrate and the epitaxial growth layer have a heterogeneous bonding structure in which different kinds of layers are bonded with each other.
However, the base substrate and the epitaxial growth layer made of heterogeneous materials have a large difference in lattice constant and thermal expansion coefficient. Therefore, many lattice defects are generated on the epitaxial growth layer due to lattice disparity between the base substrate and the epitaxial growth layer while the epitaxial grown layer is formed. In particular, the lattice defects of the epitaxial growth layer in the nitride based semiconductor device are larger in size and markedly more in the number per unit area than defects generated on the epitaxial growth layer on a silicon substrate of a silicon semiconductor device. For example, the silicon semiconductor device, in which silicon layers are grown on the silicon substrate, has a structure in which materials of the same kind of seed layer are epitaxially grown. As such, the defects of the epitaxial growth layer of the silicon semiconductor device are generated due to lattice disparity between heterogeneous materials, while defects on the silicon substrate are just transferred to the epitaxial growth layer, such that the defects of the silicon semiconductor device are generated. Contrary to this, the nitride based semiconductor device, in which a nitride layer is grown on the sapphire substrate, has a structure in which materials of a different kind from the seed layer are epitaxially grown. Most defects of the epitaxial growth layer in the nitride based semiconductor device are the defects generated due to the lattice disparity between heterogeneous materials. Such defects can be prevented from being generated if the base substrate is made of the same material or similar material as the epitaxial growth layer. Consequently, due to the defects, the production efficiency of the nitride based semiconductor device is deteriorated, leakage current is generated, and breakdown voltage is decreased. In particular, unlike a semiconductor device, in the case of the nitride based semiconductor device, since there is provided a layer where carriers are concentrated, such as the 2-DEG. Therefore, when a metal layer are formed so as to the Schottky and ohmic electrodes in the case in which many defects exist on the epitaxial growth layer, the metal layer enters the defects. In this case, the 2-DEG is electrically conducted through the metal layer in the defects, thus, causing large amounts of leakage current.